1. Field of the Invention
The present invention relates to the field of position sensors, and more specifically to a temperature compensated Hall effect actuator piston position sensor.
2. Prior Art
In certain applications it is required or at least desirable to know the position of a part without touching the part, such as the piston in a hydraulic or pneumatic actuator. By way of example, some sequential machines are designed so that the initiation of a particular operation or motion is dependent upon an appropriate signal indicating the completion of the preceding motion or operation. In other instances, it is desirable to know when the actuator is reaching the limit of its stroke for various control purposes. In robots and other systems operating under computer control, pneumatic and hydraulic actuators provide no specific feedback of position, and accordingly a degree of freedom may be suddenly lost if a limit signal is not somehow generated and provided to the computer to provide a warning of such fact. The same is not required, of course, of stepper motors and the like, as a computer may readily keep tract of all the positive and negative steps to know at all times where the stepper motor is, though similar measurements of air or hydraulic fluid flow cannot be made with sufficient accuracy to provide similar information for such actuators.
Various types of piston position sensing devices are well known in the prior art. By way of example, U.S. Pat. No. 4,176,586 discloses a system which utilizes a limit switch formed with a switch blade of ferromagnetic material located outside of the cylinder to sense the position of a piston having an annular rim constituted of permanent magnetic material. Such a system requires a special piston, as conventional actuators do not utilize permanent magnetic material pistons. A sensor in accordance with this patent would also exhibit very high hysteresis, as the motion of the switch blade will itself introduce a high degree of hysteresis. As an alternative, U.S. Pat. No. 4,071,725 discloses a proximity switch which uses a permanent magnet on the moving member of the switch external to the cylinder to respond to the presence of the magnet piston therein. In that regard, cylinders frequently utilize a steel piston and accordingly, at least this sensor would be operative with conventional actuators without requiring a special piston therein. However, this type of sensor would also exhibit very high hysteresis, primarily because of the motion of the magnet.
U.S. Pat. No. 3,364,361 discloses a piston controlled impulse generating arrangement for clamping around the cylinder of an actuator which uses a pair of windings on an iron core. One winding is connected to a source of alternating current, with a voltage induced in the second winding being indicative of the presence of the piston at that position. Alternatively, a Hall generator may be used. Such a system should have the advantage of a lower hysteresis than other prior art sensors, as the piston is the only moving part, and use of the AC current will generate an alternating field which should minimize magnetic hysteresis. However, an alternating magnetic field will not easily pass through a conductor such as an aluminum cylinder, with the eddy currents induced in the aluminum causing high stray magnetic fields and grossly limiting signal strengths. Accordingly, it is not clear that useful signals can be achieved, particularly for larger cylinders and/or thicker cylinder walls.
A final prior art device uses an externally mounted Hall effect sensor to sense the presence of a magnetic field generated by a magnetic ring attached to the piston. Again, such a system requires a special piston. In addition, because the output of a Hall effect sensor is temperature dependent, accurate sensing of the piston position can only be obtained over a very narrow temperature range.